On Tue, Mar 19, 2024 at 7:21 PM Samuel Holland <samuel.holland@xxxxxxxxxx> wrote: > > > else > > regs->status |= SR_UXL_64; > > #endif > > + current->thread_info.envcfg = ENVCFG_BASE; > > } > > > > And instead of context switching in `_switch_to`, > > In `entry.S` pick up `envcfg` from `thread_info` and write it into CSR. > > The immediate reason is that writing envcfg in ret_from_exception() adds cycles > to every IRQ and system call exit, even though most of them will not change the > envcfg value. This is especially the case when returning from an IRQ/exception > back to S-mode, since envcfg has zero effect there. > A quick observation: So I tried this on my setup. When I put `senvcfg` writes in `__switch_to ` path, qemu suddenly just tanks and takes a lot of time to boot up as opposed to when `senvcfg` was in trap return path. In my case entire userspace (all processes) have cfi enabled for them via `senvcfg` and it gets context switched. Not sure it's specific to my setup. I don't think it should be an issue on actual hardware. Still debugging why it slows down my qemu drastically when same writes to same CSR are moved from `ret_from_exception` to `switch_to`
